Mats Galbe • Guido Zacchi (И)

Dept. of Chemical Engineering, Lund University, P. O. Box 124, 221 00 Lund, Sweden Guido. Zacchi@chemeng. lth. se

1 Introduction……………………………………………………………………………………………….. 42

2 Assessment of Pretreatment…………………………………………………………………………. 44

3 Pretreatment Methods…………………………………………………………………………………. 47

3.1 Physical Methods………………………………………………………………………………………… 48

3.2 Chemical Methods………………………………………………………………………………………. 48

3.3 Physicochemical Methods…………………………………………………………………………….. 49

3.4 Biological Methods………………………………………………………………………………………. 51

4 Results from Pretreatment Studies………………………………………………………………. 51

4.1 Corn Stover…………………………………………………………………………………………………. 52

4.2 Softwood Species………………………………………………………………………………………… 57

4.3 Two-Stage Pretreatment………………………………………………………………………………. 60

5 Conclusions………………………………………………………………………………………………… 62

References……………………………………………………………………………………………………. 62

Abstract Second-generation bioethanol produced from various lignocellulosic materials, such as wood, agricultural or forest residues, has the potential to be a valuable substitute for, or a complement to, gasoline. One of the crucial steps in the ethanol production is the hydrolysis of the hemicellulose and cellulose to monomer sugars. The most promising method for hydrolysis of cellulose to glucose is by use of enzymes, i. e. cellulases. However, in order to make the raw material accessible to the enzymes some kind of pretreatment is necessary. During the last few years a large number of pretreatment methods have been developed, comprising methods working at low pH, i. e. acid based, medium pH (without addition of catalysts), and high pH, i. e. with a base as catalyst. Many methods have been shown to result in high sugar yields, above 90% of theoretical for agricultural residues, especially for corn stover. For more recalcitrant materials, e. g. softwood, acid hydrolysis and steam pretreatment with acid catalyst seem to be the methods that can be used to obtain high sugar and ethanol yields. However, for more accurate comparison of differ­ent pretreatment methods it is necessary to improve the assessment methods under real process conditions. The whole process must be considered when a performance evalua­tion is to be made, as the various pretreatment methods give different types of materials. (Hemicellulose sugars can be obtained either in the liquid as monomer or oligomer sug­ars, or in the solid material to various extents; lignin can be either in the liquid or remain in the solid part; the composition and amount/concentration of possible inhibitory com­pounds also vary.) This will affect how the enzymatic hydrolysis should be performed

(e. g. with or without hemicellulases), how the lignin is recovered and also the use of the lignin co-product.

Keywords Assessment • Enzymatic hydrolysis • Lignocellulose • Pretreatment • Review

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Introduction

Replacement of gasoline by liquid fuels produced from renewable sources is a high-priority goal in many countries worldwide. It is driven by the aims of a secure and sustainable energy supply and a desire to diminish the green­house effect. The transportation sector in the European Union (EU) is totally dependent on imported fossil fuels, and thus extremely vulnerable to market disturbances. It is also the sector responsible for the main part of the increase in CO2 emissions. The use of biofuels in the EU is encouraged by a Directive that set a target of 2% substitution of gasoline and diesel with biofuels in 2005 on an energy basis, which should have increased to 5.75% by 2010 [1].

Bioethanol is projected to be one of the dominating renewable biofuels in the transportation sector within the coming 20 years, and has already been introduced on a large scale in Brazil, the USA and some European countries. The advantages of bioethanol are that it can be produced from a variety of raw materials, it is non-toxic and is easily introduced into the existing in­frastructure, either as a low blend with gasoline (e. g. E5 and E10) or used in flexi-fuel vehicles at a high concentration (e. g. E85) or as a neat fuel in dedi­cated engines. However, almost all bioethanol today is produced from sugar — or starch-based agricultural crops, using so-called first-generation technolo­gies. Although this ethanol is produced at a competitive cost, the raw material supply will not be sufficient to meet the increasing demand for fuel ethanol, and also the reduction of greenhouse gases resulting from the use of sugar — or starch-based ethanol is not as high as desirable.

One of the most promising options to meet this challenge is the production of bioethanol from lignocellulose feedstocks, such as agricultural residues (e. g. wheat straw, sugar cane bagasse, corn stover) and forest residues (e. g. sawdust, thinning rests), as well as from dedicated crops (salix, switch grass) using second-generation technologies. These raw materials are sufficiently abundant and generate very low net greenhouse gas emissions, reducing en­vironmental impacts.

However, to compete with gasoline the production cost must be substan­tially lowered. Today, raw material and enzyme production are two of the main contributors to the overall production cost [2,3]. Efficient use of the whole crop is required, i. e. high overall yield of ethanol produced by hydro­lysis and fermentation of the carbohydrate fraction (hemicellulose and cel­lulose), as well as a high yield of the main co-product (lignin). However,
producing monomer sugars from cellulose and hemicellulose at high yields is far more difficult than deriving sugars from sugar — or starch-containing crops, e. g. sugar cane or corn.

Ethanol production from lignocellulosic materials comprises the following main steps: hydrolysis of hemicellulose; hydrolysis of cellulose; fermenta­tion; separation of lignin residue; recovery and concentration of ethanol; and wastewater handling. Figure 1 shows a simplified process flowsheet for ethanol production from lignocellulosic materials based on enzymatic hydro­lysis. Some of the most important factors to reduce the production cost are: an efficient utilization of the raw material by having high ethanol yields, high productivity, high ethanol concentration in the distillation feed, and also by employing process integration in order to reduce capital cost and energy de­mand. Part of the lignin can be burnt to provide heat and electricity for the process, and the surplus is sold as a co-product for heat and power appli­cations, which will increase the energy efficiency of the whole system. It is thus necessary to minimize the internal energy demand and to maximize the production of the solid fuel. The two conversion steps can be considered key processes: hydrolysis of the hemicellulose and cellulose to sugars, and fer­mentation of all the sugars.

The enzymatic process is regarded as the most attractive way to degrade cellulose to glucose [4-6]. However, enzyme-catalysed conversion of cellulose to glucose is very slow unless the biomass has been subjected to some form of pretreatment, as native cellulose is well protected by a matrix of hemicel­lulose and lignin. Pretreatment of the raw material is perhaps the single most crucial step as it has a large impact on all the other steps in the process, e. g. enzymatic hydrolysis, fermentation, downstream processing and wastewater handling, in terms of digestibility of the cellulose, fermentation toxicity, stir­ring power requirements, energy demand in the downstream processes and wastewater treatment demands.

An effective pretreatment should have a number of features. It has to:

• Result in high recovery of all carbohydrates.

• Result in high digestibility of the cellulose in the subsequent enzymatic hydrolysis.

• Produce no or very limited amounts of sugar and lignin degradation prod­ucts [7]. The pretreatment liquid should be possible to ferment without detoxification.

• Result in high solids concentration as well as high concentrations of liber­ated sugars in the liquid fraction.

• Have a low energy demand or be performed in a way so that the energy can be reused in other process steps as secondary heat.

• Have a low capital and operational cost.

Additional positive features would be if hemicellulose sugars were obtained in the liquid as monomer sugars, as this would help to avoid the use of hemi — cellulases, and/or if the lignin—without being oxidized—was separated from the cellulose, as this would alleviate the unproductive binding of cellulases on lignin in the enzymatic hydrolysis step.

This chapter will focus on pretreatment of lignocellulosic raw materials. Some of the most common methods that have been investigated will be pre­sented and to some extent compared for various raw materials.

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